1. Field of the Invention
This invention relates to an image forming apparatus.
2. Description of the Related Art
In an image forming apparatus that forms an image by electronic photography process, such as printer, copier, facsimile, and multi-function peripheral thereof, a toner image is developed on a photoconductor drum, and the toner image is transferred onto the intermediate transfer belt, and then transferred from the intermediate transfer belt to a sheet of print paper, and finally the toner image is fixed on the sheet of print paper.
In such image forming apparatus, when necessary or periodically, toner density and its gradation are adjusted. In four-color image forming apparatus, toner density and its gradation are adjusted for each of four colors.
To print a scanned document, some image forming apparatuses chooses a threshold conversion process (e.g. an error diffusion process or a screen process) according to the type of the document, and generate image data by using the chosen process. Otherwise, for a single page, a plural of threshold conversion processes may be used. Therefore, gradation adjustment must be performed for the error diffusion process and the screen process separately.
In the case that the apparatus performs automatic gradation adjustment of the error diffusion process, for instance, the apparatus has a ROM (Read Only Memory) that stores data of a pattern image generated by a single-threshold conversion from patch images with density values in the gradation, and forms a toner pattern image based on the data, detects the pattern image by a sensor, and then performs gradation adjustment.
Alternatively, the apparatus performs the error diffusion process for each of pixels sequentially to generate data of a pattern image of single-threshold conversion, and stores the generated data in a RAM (Random Access Memory); and then forms a toner pattern image based on the data, detects the pattern image by a sensor, and then performs gradation adjustment.
In case of using a threshold conversion method such as error diffusion process that performs threshold conversion (e.g. single threshold conversion) by sequentially calculating a converted pixel value from original values of adjacent pixels, since a pattern image for adjustment generated by such method is irregular (i.e. has a long cycle), large capacity is required to a ROM or a RAM that stores the pattern image. Therefore, a cost of the apparatus tends to be high.
It should be noted that a gradation characteristic of one of an error diffusion process and a screen process can be adjusted according to the other of them. However, there are large difference between the error diffusion process and the screen process in half-toning calculation, and the gradation characteristics of them do not vary in the same manner due to its usage environment and its usage situation, and consequently, such adjustment may not be accurate.